1. Field of the Invention
The present invention relates to a reference voltage generation circuit mounted on a semiconductor device, for generating a reference voltage.
2. Description of the Related Art
In recent years, advanced electronic devices have been marketed all over the world and used in various environments. For example, electronic devices are used also in frigid and heavy snowfall areas, tropical areas right on the equator, and other areas. The electronic devices are required to operate normally in almost any temperature environment of human life. Characteristics of a semiconductor device mounted on an electronic device are thus required not to change due to temperature. One reason for deterioration of temperature characteristics of such semiconductor device is that a reference voltage generated by a reference voltage generation circuit in the semiconductor device changes due to temperature.
A related-art reference voltage generation circuit is described with reference to FIG. 4.
The reference voltage generation circuit includes a depletion type NMOS transistor (D-type NMOS transistor) 91 and an enhancement type NMOS transistor (E-type NMOS transistor) 92. The D-type NMOS transistor 91 has a gate connected to a source thereof so as to function as a constant current circuit. The E-type NMOS transistor 92 is diode-connected. Those transistors are connected in series between a power supply terminal and a ground terminal. The D-type NMOS transistor 91 supplies a constant current to the E-type NMOS transistor 92. This constant current generates a reference voltage VREF at a drain of the E-type NMOS transistor 92.
In this case, the reference voltage VREF is determined by threshold voltages of those transistors and the sizes thereof. Japanese Patent Application Laid-open No. Sho 59-200320 (FIG. 3 and Expression (3)) describes that, by adjusting the sizes of those transistors, temperature dependence of the reference voltage VREF can be reduced.
However, the reference voltage generation circuit disclosed in Japanese Patent Application Laid-open No. Sho 59-200320 has a problem in that the threshold voltages of the transistors may fluctuate due to fluctuations in manufacturing step in a semiconductor manufacturing process and the reference voltage VREF may change due to temperature.